1. Field of the Invention
The present invention relates to a downlink transmission power control scheme for realizing an efficient transmission power control with respect to downlink radio signals which are transmitted from base stations to a mobile station via radio channels in a mobile communication system using a site diversity.
2. Description of the Background Art
A radio communication scheme can incorporate a transmission power control for the purpose of suppressing a transmission power to a minimum necessary level. By carrying out this transmission power control, it is possible to realize such advantageous effects as a saving of power consumption and a reduction of interferences with respect to other radio channels. In particular, in the CDMA (Code Division Multiple Access) scheme, a lower amount of interferences directly implies a larger subscriber capacity, so that the transmission power control is an indispensable technique.
On the other hand, one known technique for reducing interferences in the CDMA scheme is the site diversity, which is a technique in which a mobile station and a plurality of base stations are simultaneously connected and a diversity combining is carried out among a plurality of base stations. This site diversity is capable of satisfying a given communication quality by using a less transmission power, so that the interferences can be reduced and the subscriber capacity can be increased.
Now, in the radio communication in general, the propagation loss is not identical for the uplink channel from a mobile station to a base station and the downlink channel from a base station to a mobile station. Consequently, in order to improve the transmission power control precision, it is preferable to carry out the closed loop transmission power control.
In the closed loop transmission power control, a base station transmission power in a downlink communication channel is controlled by a transmission power control signal according to a receiving quality measured at a mobile station as indicated in a part (b) of FIG. 1, while a mobile station transmission power in an uplink communication channel is controlled by a transmission power control signal according to a receiving quality measured at a base station as indicated in a part (a) of FIG. 1.
For example, such a closed loop transmission power control can be realized by using one bit information as follows. Namely, a receiving side transmits to a transmitting side a transmission power control command which indicates "0" when the receiving quality measured at the receiving side does not satisfy a required quality, or "1" when the receiving quality measured at the receiving side satisfies the required quality. Then, at the transmitting side, the transmission power is raised by one step when the transmission power control command indicates "0", or lowered by one step when the transmission power control command indicates "1". By carrying out this control continuously, it is possible to maintain the receiving quality nearly at a level of the required quality.
This closed loop transmission power control can be realized by using either a control signal which is terminated between a mobile station and a base station, or a control signal which is terminated between a mobile station and a base station control station. In the latter case, the control signal transmitted by a mobile station is received by a plurality of base stations-and combined during a site diversity period, and then transmitted to a base station control station.
The control signal terminated between a mobile station and a base station is usually transmitted on layer-1, and will be referred to as the layer-1 control signal. The control signal terminated between a mobile station and a base station control station is usually transmitted on layer-3, and will be referred to as the layer-3 control signal.
In a case of realizing the closed loop transmission power control, a higher transmission power control precision can be realized by a shorter transmission power control period. From this point of view, it is more convenient to transmit the transmission power control signal as the layer-1 control signal, because the layer-1 control signal does not require the encoding processing or the re-transmission processing so that a very fast transmission power control can be realized.
FIG. 2 shows an exemplary case of the downlink transmission power control using the layer-1 control signal. In this case, a loop is formed between a mobile station and a base station, so that it is possible to realize a fast transmission power control with a small control delay and therefore it is possible to reduce a transmission power control error.
However, the layer-1 control signal is to be received independently by each base station, so that there is a problem in realizing the control during a site diversity period. Namely, when a control for maintaining a quality after the site diversity combining above a certain level is carried out in the uplink channel, it becomes impossible to maintain a sufficient uplink receiving quality at some base station among a plurality of base stations that are simultaneously connected with a mobile station, so that there is a possibility for raising an error rate of the layer-1 control signal transmitted from a mobile station to a base station. At such a base station, the transmission power control error becomes large so that an amount of interferences is increased and this in turn causes a lowering of a capacity in a case of the CDMA mobile communication system.
FIG. 3 shows an exemplary case of the downlink transmission power control using the layer-3 control signal. In this case, during a site diversity period, the layer-3 control signal is combined at an exchange station and then transmitted to a base station control station. Consequently, the reliability of the layer-3 control signal is quite high and the transmission powers of the base stations are always controlled uniformly according to the same information.
However, due to the transmission delay of the layer-3 control signal, it is impossible to realize a fast transmission power control, and a capacity is lowered as the transmission power control error becomes large. In addition, there is also a problem that an amount of control signals to be transmitted between a base station and a base station control station is increased.
Conventionally, the closed loop transmission power control has been realized either by always using the layer-3 control signal as in a case of FIG. 3, or by always using the layer-1 control signal as in a case of FIG. 2.
Thus, when the transmission power control by always using the layer-1 control signal is carried out in a mobile communication system using the site diversity, there arises a problem of an increased transmission power control error during a site diversity period. On the other hand, when the transmission power control by always using the layer-3 control signal is carried out in a mobile communication system using the site diversity, there arises a problem of an increased transmission power control error and an increased amount of signals transmitted between stations during a non-site diversity period. Here, an increased transmission power control error can cause an increase of interferences which in turn can cause a lowering of a capacity in the CDMA mobile communication system.